1. Field of the Invention
The present invention relates to method and apparatus for processing polarization data of polarization sensitive optical coherence tomography.
2. Description of the Related Art
Optical coherence tomography (hereinafter referred to as OCT) using multiple-wavelength optical interference can provide a high-resolution tomographic image of a sample (ocular fundus, in particular). In recent years, OCT apparatuses for ophthalmology have been studied to acquire a polarization sensitive OCT image for imaging the polarization parameter, which represents one of optical properties inherent to fundus tissues, in addition to an ordinary OCT image for imaging the shape of the fundus tissues.
By using polarization parameters, the polarization sensitive OCT apparatus can take a polarization sensitive OCT image and perform characteristic measurement or segmentation of the fundus tissues. The polarization sensitive OCT apparatus uses a beam that is modulated into a circularly-polarized beam as a measuring beam for observing a sample, and splits an interference beam into two linearly-polarized beams of orthogonal polarization states and detects the resulting two beams so as to generate polarization sensitive OCT images (see “E. Goetzinger et al, “Speckle noise reduction in high speed polarization sensitive spectral domain optical coherence tomography”, Optics Express. 19(15), 14568-14584” (NPL 1)). Further, the polarization sensitive OCT is capable of imaging retardation as one of the polarization parameters, that is defined as phase difference between two polarized beam components. The retardation is useful to detect changes in a retinal nerve fiber layer for diagnosing glaucoma.
The literature further discloses a method of reducing the speckle noise peculiar to coherent light used for OCT by using plural polarization sensitive OCT images. According to the method, the speckle noise is reduced by averaging retardations that are obtained through the polarization sensitive OCT. As a result, the graininess of a resultant retardation image is significantly improved.
Retardation is defined as phase difference between a fast axis and a slow axis, which is observed when light passes through a medium. Therefore, it is useful to perform averaging considering angles which the fast axis and the slow axis form with the axis of the light used in an OCT apparatus (hereinafter referred to as an axis orientation). However, according to the method disclosed in NPL 1, the retardation is obtained on the basis of calculation of the arctangent of a ratio between the two polarized components (retardation=arctan(I1/I2)) and the retardation is obtained without referring to the axis orientation. Therefore, the value of the retardation necessarily falls within a range of 0 to 90°. This method uses only intensity of the two polarized components and noise has some intensity level that takes zero or positive value. For this reason, averaging of the noise does not converge to zero and introduces residual or offset. This offset in the case of low retardation value causes an undesirable artifact on an image. Thus, even in a case where a small signal is observed, when the retardation should approach 0, noise causes the retardation to have a non-zero value sometimes referred to a retardation offset. In other words, by measuring retardation using intensities of the polarized components (as the conventional method does), noise in those intensities causes artifacts in the retardation value.